JP2010515067A - Micro spectrometer gas analyzer - Google Patents

Abstract

  A rugged and compact spectrometer system for determining the concentration and partial pressure of each of multiple gases in a gas sample with a single and even overlapping multiple absorption or emission spectra that scan a wide spectral range . For example, an infrared light source for emitting an infrared beam, a gas sample cell disposed in the path of the infrared beam, and a plurality of parallel lines disposed in the path of the infrared beam after passing through the gas sample cell A scanning mirror having a diffraction grating, a resonant scanner drive system for oscillating the scanning mirror about an axis parallel to the line of the diffraction grating, and focusing at least one related band of the infrared beam diffracted by the diffraction grating A first focusing element arranged to receive, a first detector arranged to receive the focused at least one related band, and the first detector to receive a signal from the first detector A spectrometer having a first detector readout circuit operatively coupled to the is disclosed.

Description

  The present invention relates to a method and apparatus for efficiently and powerfully measuring the gas concentration / partial pressure of respiratory and anesthetic gases.

  Non-dispersive infrared (NDIR) type gas analyzers allow specific gas concentrations to (a) direct infrared radiation (IR) through a sample of a gaseous mixture and (b) be absorbed by each specific gas Filter this infrared radiation separately to minimize out-of-band energy, (c) measure the filtered radiation that strikes one or more detectors, and (d) the degree of infrared absorption of each gas. Works on the principle that it can be determined by relating it to its concentration. Gases that can be measured show increased absorption (and decreased transmission) at specific wavelengths in the infrared spectrum, with higher concentrations of gas increasing absorption and decreasing transmission. . This extension of the NDIR technique uses a continuous linear bandpass filter followed by a linear array of detectors.

  Gas analyzers are widely used in medical applications and are installed in the main path of patient breathing gas (mainstream analyzer) or in a subpath that is normally parallel to the main path (substream analyzer). It is characterized by that. The mainstream analyzer is positioned such that the subject's inspiratory and expiratory respiratory gases pass through the airway adapter in which the analyzer is positioned. Mainstream designs require optical and electrical components that are interfaced to the patient's airway or to a breathing circuit that communicates with the patient located relatively close to the patient. As a result, to be accepted for clinical use, mainstream gas analyzers are smaller, lighter, and more robust structures that are not affected by typical mechanical misuse or temperature fluctuations associated with long-term use in health care facilities Must be designed as a body.

  Conventional mainstream gas analyzers work well for a small number of specific non-overlapping spectral wavelengths, but it is difficult to change the wavelength of interest. When there are two or more wavelengths of interest, the system becomes increasingly inefficient and it is very difficult and expensive to provide significantly better resolution than 0.1 micron FWHM in the IR region It is.

  It is known to use a grating spectrometer for gas analysis. There are two general configurations for grating spectrometers. One is a spectrograph, which originally develops a spectrum on a strip of photographic film or a linear array detector. Another is a spectrometer, which uses a single detector placed at the appropriate position or angle to record a particular spectral component.

  For IR gas measurements, the IR light source provides broadband energy that is collimated and passes through the gas sample cell. The collimated broadband energy is then attenuated at a specific wavelength and directed to a diffraction grating where it is diffracted from the diffraction grating and spread into a continuous spectrum that is focused by a mirror onto a small detector. The The diffraction grating rotates about an axis that is parallel to the grating lines and substantially coaxial with the plane of the diffraction grating. As the diffraction grating rotates, the spectrum is scanned through a single detector. Since the rotation of the grating is synchronized with the detector readout electronics, certain (but optional) spectral features can be recorded separately.

  Obviously, a micro spectrometer must be small and light. For example, the present invention contemplates that the micro-spectrometer is made small and lightweight enough to be used directly on the patient airway, i.e. mounted in a mainstream manner on the patient circuit. Optical components can generally be made small enough to be suitable for this purpose, but the mechanism that drives the diffraction grating (ie spectrum scanner) should be made small enough to be suitable for this purpose. Is difficult. Currently available electromechanical scanner drives are very large and in most cases too heavy and require very much power and are very expensive to use in this way.

  For example, many conventional spectrometers rotate the diffraction grating using some type of motor, a vibration coupling that drives the diffraction grating from the motor, and a bearing assembly. Such a device can achieve good results, but such a structure is relatively large, heavy and expensive. Other conventional spectrometers use a vibration motor, sometimes called a galvanometer drive, instead of a motor and connection. Such devices are somewhat inexpensive, but are still large, heavy, and relatively expensive.

  U.S. Pat. Although disclosed, the design is very bulky and in some cases very complex.

  Accordingly, it is an object of the present invention to provide a spectrometer that overcomes the disadvantages of conventional gas analyzers. The purpose of this is to provide a powerful spectrometer device for determining the concentration or partial pressure of each of a plurality of gases in a gas sample with a single and even multiple overlapping absorption or emission spectra that span a broad spectral region. By providing, this is achieved by one embodiment of the present invention.

  The present invention adapts a grating spectrometer for a small respiratory gas analyzer. Specifically, the present invention uses a scanning spectrometer that scans or sweeps the spectrum across a fixed detector. From an optical point of view, this device can be characterized as an improved Evert scanning spectrometer.

  Very small and inexpensive oscillating mirrors can be manufactured using a MEMS (MicroElectroMechanical System) manufacturing process. With a diffraction grating added to the mirror surface, this structure provides a scanner for an in-line IR gas analyzer that is very low cost, small, lightweight but robust.

  Spectral resolution is mainly a function of grating size, aperture, line pitch, diffraction order and collimation. In the present invention, the required grating width is in the range of 1-2 mm, which is well suited to existing MEMS technology. The other parameters are achieved or controlled adequately and easily enough for at least the required accuracy.

  The diffraction grating can be formed separately and glued onto the “mirror” surface, or preferentially, the diffraction grating can be formed on the surface of the mirror as part of the MEMS manufacturing process. Hologram type gratings can be used. The driving force that oscillates the mirror can be magnetic, and the mirror can have a planar coil formed on its back surface, the mirror itself can be magnetized, or the mirror can be driven electrostatically. it can. Electrostatic driving is generally preferred because the required angular amplitude is relatively small.

  The apparatus of the present invention can also be configured in several further aspects. In one case, the oscillating grating can be removed and replaced by a scanning (oscillating) mirror. In an embodiment of this approach, the mirror scans the input light on a fixed grating that disperses the spectrum. As already mentioned, the spectrum is focused on the detector surface by a mirror. Although this alternative method requires one additional component, the MEMS oscillator element does not need to have a grating manufactured on its surface, which can reduce manufacturing costs.

  In yet another embodiment, the oscillating mirror can be arranged to direct the attenuated broadband energy beam back through the gas sample cell, and the grating and detector can be connected to an IR light source with respect to the gas sample cell. On the same side. The advantage of this arrangement is higher sensitivity (due to the double optical path through the gas in the cell) and a somewhat narrower package. Alternatively, in a double optical path configuration, the mirror opposite the light source can be fixed, and the oscillating mirror / fixed grating (or oscillating grating) and detector system are located on the light source side. These various embodiments can be configured in a single plane, or the oscillating mirror, scanning grating or focusing mirror can be rotated to direct the beam in different planes, and in different packages A configuration can be easily provided.

  The diffraction grating can provide several orders of diffraction beams. Usually, the first order ± 1 is used, and the shape of the groove in the grating is designed to highlight the selected order. However, there may be some residual energy in higher orders. As a result, the short wavelength spectral region may overlap the primary spectrum. This problem can be solved by a cut-off filter that is set to cut off all wavelengths outside the spectral region of interest, if desired.

  The data processing electronics for the device of the present invention are synchronized with the operation of the scanning element. One approach is to extract the timing signal from the mirror drive. Alternatively, the mirror may comprise a coil or magnetic or piezoelectric sensor mounted thereon to provide a signal indicative of a substantially instantaneous position of the portion of the mirror for use in synchronization. Another sensing technique for use in synchronization is to reflect the auxiliary beam at the front or back of the mirror to another detector. A generally preferred technique is to use the unique characteristics of the detected spectrum when such is available or provided. If the mirror is assumed to resonate, the period during which the detector receives no signal is relatively long. This is because it is easier to interpret when the scan is in the approximately linear part of the scan, and the cutoff filter removes all signals before or after the spectral part of interest. Thus, a long blank period after which the signal sharply increases can be used to provide an appropriate unique marker for the phase locked loop synchronizer. The blank period provides background light conditions so that detector zero can be set. Full scale can be indicated by any spectral region between absorption peaks or regions where known peaks have been subtracted.

  Since the data generated by the device is continuous, it is considered possible to sequentially subtract one known and previously stored specific spectral lines one by one, ie "stripping" individual lines. Yes. Such processing improves separation or reduces particularly weak line interference.

  These and other objects, features and characteristics of the present invention, as well as the function of the relevant elements of the method of operation, structure and combination of components, as well as the economy of manufacture, are described in the following description and attached with reference to the accompanying drawings. It will become more apparent in light of the appended claims. In each figure, like reference numerals refer to corresponding parts. It should be expressly understood, however, that the drawings are for purposes of illustration and description only and are not intended to define the scope of the invention.

1 is a schematic optical system layout for a spectrometer having a combination of an oscillating scanner mirror and a diffraction grating according to the principles of the present invention. FIG. 1B is a schematic diagram of a spectrometer in which the optical system of FIG. 1A can be suitably used. FIG. 1B is a perspective view of an oscillation mirror / grating combination suitable for use in the optical system of FIG. 1A. Fig. 2 is a schematic optical system layout for a spectrometer having a combination of a focusing mirror and a diffraction grating according to the present invention. FIG. 3 is a schematic diagram of an exemplary layout for a spectrometer that allows analysis of multiple spectral bands using a collimated light beam in accordance with the principles of the present invention. FIG. 3 is a schematic diagram of an exemplary layout for a spectrometer that allows analysis of multiple spectral bands using a collimated light beam in accordance with the principles of the present invention. FIG. 3 is a schematic diagram of an exemplary layout for a spectrometer that allows analysis of multiple spectral bands using a collimated light beam in accordance with the principles of the present invention. FIG. 3 is a schematic diagram of an exemplary layout for a spectrometer that allows analysis of multiple spectral bands using a collimated light beam in accordance with the principles of the present invention. FIG. 3 is a schematic diagram of an exemplary layout for a spectrometer that allows analysis of multiple spectral bands using a collimated light beam in accordance with the principles of the present invention. FIG. 3 is a schematic diagram of an exemplary layout for a spectrometer that allows analysis of multiple spectral bands using a collimated light beam in accordance with the principles of the present invention. FIG. 4 is a schematic diagram of an exemplary layout for a spectrometer that allows analysis of multiple spectral bands using a non-collimated light beam in accordance with the principles of the present invention. FIG. 4 is a schematic diagram of an exemplary layout for a spectrometer that allows analysis of multiple spectral bands using a non-collimated light beam in accordance with the principles of the present invention. FIG. 4 is a schematic diagram of an exemplary layout for a spectrometer that allows analysis of multiple spectral bands using a non-collimated light beam in accordance with the principles of the present invention. FIG. 3 is a schematic diagram of a further exemplary arrangement for a spectrometer according to the principles of the present invention. FIG. 3 is a schematic diagram of a further exemplary arrangement for a spectrometer according to the principles of the present invention. FIG. 3 is a schematic diagram of a further exemplary arrangement for a spectrometer according to the principles of the present invention. FIG. 3 is a schematic diagram of a further exemplary arrangement for a spectrometer according to the principles of the present invention. 1 is a top perspective view of an electromechanical scanner drive according to the principles of the present invention. FIG. 1 is a bottom perspective view of an electromechanical scanner drive according to the principles of the present invention. FIG. 1 is a schematic diagram of a circuit for performing automatic scan frequency adjustment in accordance with the principles of the present invention. Waveform showing return signal for scanner drive during resonance. A waveform showing a return signal for driving the scanner during non-resonance.

  FIG. 1A is a schematic optical layout for a spectrometer according to the principles of the present invention. Energy in the form of a light beam 10 (for example, an infrared beam) is emitted from the sample cell G (see FIG. 1B) and strikes the reflecting mirror 12. The reflecting mirror 12 reflects the light beam 10 toward the scanning grating reflector 14, which may be called a scanning mirror. Note that the scanning grating reflector 14 oscillates about an axis perpendicular to the page (oscillation is shown in exaggerated form). From the scanning grating reflector 14, the now split light beam 10 propagates to a focusing mirror 16, which then focuses the light beam 10 onto a detector 18 that includes or is coupled to a suitable readout circuit. Bundle. The detector 18 can comprise, for example, a detector defined by slits or pinholes, as is known in the prior art.

  FIG. 1B schematically illustrates the completed structure of the spectrometer used in the various optical embodiments of the present invention. As shown in FIG. 1B, the infrared light source S emits an infrared beam that can be collimated as shown using light source optics or collimator C. The collimated infrared beam then enters the gas sample cell G and exits the cell towards the reflector 12. Such an arrangement is described herein except that it should be noted that the embodiment of FIGS. 5A-5C does not require the presence of a collimator C or light source optics to collimate the infrared beam. It can be used by all the embodiments to be made.

  Referring to FIG. 2, the scanning grating reflector 14 has a diffraction grating line 22 disposed thereon. These lines can be glued onto the reflective surface of the mirror or machined using a MEMS process, or they can be placed in some other known technique. US Pat. No. 6,201,269 (McClelland, et al., The disclosure of which is incorporated herein by reference) discloses a MEMS process suitable for the fabrication of oscillating mirrors, which includes a scanning grating reflector 14. It can be used for manufacturing. The grating can also be produced in the form of a hologram.

  The scanning grating reflector 14 has a flexible shaft 24 parallel to the diffraction line 22 and is mounted on the frame 26 through a support member coaxial with the flexible shaft 24. As is well known in the art, the backing 28 is conductive so as to provide electrostatic drive to the scanning grating reflector 14 when the conductor 20 is connected between the backing 28 and a suitable power supply P. It is. For simplicity of explanation, two power supplies P are shown in FIG. 2, but of course a single power supply P can be used to alternately supply power to the backing 28.

  The schematic diagram shown in FIG. 1A uses a scanning grating reflector 14 as both a scanner and a diffraction grating. However, it is not necessary to include a diffraction grating in the scanner. The diffraction grating can instead be scanned in angle by a mirror scanner. As shown in FIG. 3, a mirror scanner 32 is used to sweep the input beam 30 from the gas sample cell over the diffraction grating and mirror combination 34. The mirror used in the diffraction grating and mirror combination 34 is a focusing element that guides and disperses the dispersed energy from the mirror scanner 32 to the detector 36. The image formed is a defined input aperture at a wavelength selected by the diffraction grating and mirror combination 34. In a conventional Evert spectrometer, a slit that defines an aperture to be imaged is at the entrance to the spectrometer. In the present invention, the defined aperture can be a light source or can be another aperture near the entrance to the scanner / detector assembly. The reflector 12 of the embodiment of FIG. 1A does not have a structural replica in FIG. This reflector is not a required component of the present invention, but is common in the prior art and its use allows for several other configurations.

  As another alternative, the functions of the mirror and the grating can be separated, the scan is directed to a flat grating mirror, followed by a focusing element (usually this infrared wavelength region mirror), and then a detector . The advantage of such an alternative split configuration over the configuration of FIG. 1A is that although it is unusual to form the grating on the mirror, the scanning mirror device can be manufactured directly by currently known processes. In contrast, it is common to form a grating on a focusing element by a molding technique. The disadvantage of the split configuration is that the grating must be somewhat larger (since the beam moves across the grating to change the angle), and the mirror may need to be aspheric. . As expected, these are minor issues when grating mirrors are made by a molding or casting process.

  The embodiments described with respect to FIGS. 1A and 3 provide an effective way to collect spectral data over wavelength octaves. However, these embodiments are designed with one band (eg, 3-5 micron band) in mind.

  In practice, the range of the grating spectrometer is limited to octaves due to multiple orders. That is, a particular wavelength diffracts at a particular set of angles determined by an integer known as that wavelength, grating period, and order. Since dispersion is a function of order, multiple orders can overlap on the detector plane, making spectrum interpretation difficult. In a practical grating spectrometer, the grating is fabricated so that most of the diffracted energy is directed to a specific desired order. This is done by forming a surface in each groove of the diffraction grating so that light falling on that point is reflected in the same direction as the desired diffraction order. This forming process is called Blazing. In addition, a blocking filter can be added to the spectrometer input or detector to block wavelength regions that could otherwise cause confusion.

  In addition to the 3-5 micron band previously described, it is advantageous that the present invention simultaneously measures the 7-10 micron range. The problems in this longer wavelength band are firstly the need for more expensive detectors, and secondly, transmission optics for beam manipulation (although long wavelength transmission filters or functions are inevitable). (E.g. lenses) tend to be more expensive and third, the second order of the 3-5 micron band tends to fall into the same plane as the 7-10 micron band.

  Seven exemplary approaches to optical placement for further band measurements are shown in FIGS. 4A-4F. In all illustrated embodiments shown in FIGS. 4A-4F, the input beam is already collimated by the light source optics or other conventional means. Furthermore, the drawings are schematic, i.e., the diffraction angles are illustrative and not accurate.

  In the embodiment of FIG. 4A, scanning mirror 42 directs input beam 40 to dichroic beam splitter 44, which splits the beam into two bands (eg, 3-5 and 7-10 microns), respectively. Two separate scanning diffraction gratings 46 disperse the above bands. Each grating 46 is optimized for each band. After dispersion, each band of the beam is directed by focusing mirror 48 onto the aperture of detector D.

  In the embodiment of FIG. 4B, a scanning diffraction grating 46 is used and the resulting dispersed beam is split into two bands by a dichroic beam splitter 44. In this case, the scanning grating 46 is optimized in the first order for the 7-10 micron band and in the second order for the 3-5 micron band.

  FIG. 4C shows an embodiment that includes a scanning mirror 42 followed by a dichroic diffraction grating 47 that is coated to reflect one band (eg, 7-10 microns) and transmit the other band. In other cases, the dichroic grating 47 is tuned for a first order of 7-10 microns and a second order of 3-5 microns. Alternatively, a reflective diffraction grating (non-transmissive) can be used and a band splitter is placed after this diffraction grating.

  The embodiment of FIG. 4D uses a back-to-back scanning diffraction grating 46 that only reflects and is used as a scanning element. The band division is performed by the dichroic beam splitter 44 before the grating. In this embodiment, the grating can be individually optimized for best performance in a particular band.

  The embodiment of FIG. 4E is tuned to provide detection in three bands. The scanning mirror 42 sequentially illuminates the two reflection / transmission dichroic diffraction gratings 47. This arrangement causes some limitations with respect to the wavelength band arrangement, but is physically smaller than the arrangement of FIG. 4F.

  The embodiment of FIG. 4F includes a three-dimensional arrangement of mirrors and gratings that can provide six bands (as shown) and more bands with extensions. Input beam 50 is first split into three wavelength blocks of two adjacent octave bands, each using a plurality of dichroic or bandpass filters 51, and the wavelength blocks are scanned by scanning mirror 52. The axis of the scanning mirror 52 is in the plane of the drawing sheet. Wavelength blocks are geometrically separated by the angle of the plane containing the mirror rotation axis. After scanning, the wavelength block proceeds to three diffraction gratings 56, each similar to the diffraction grating of FIG. 4C, but appropriately tilted to fit the separation angle. In FIG. 4F, for the sake of brevity and clarity, only one grating 56 is shown and no detector is shown, but such is actually included.

  FIGS. 5A-5C illustrate a further embodiment of the present invention, unlike the previous embodiment, the light entering the spectrometer can be diverging or converging and the optical component compensates for it Will be changed as follows.

  FIG. 5A schematically shows a system in which light from a light source S passes through a gas sample cell G, is reflected, dispersed by a grating, and scanned by a scanning plane grating mirror 60. The resulting dispersed light beam is focused onto detector D using concave mirror 62.

  FIG. 5B schematically illustrates a system using a planar scanning mirror 64, where the scanned beam is reflected to a concave grating mirror 66 that diffracts the light beam and focuses it onto detector D.

  FIG. 5C schematically illustrates a system in which scanning, dispersion and focusing functions are integrated into one element 68 in the form of a scanning mirror that is concave to focus the light beam onto detector D, including a diffraction grating. Shown in

  As understood and appreciated by those skilled in the art, adding functionality to the scanning element increases its cost, but in each case, other elements in the system can be reduced in cost. Can be omitted altogether. In particular, the embodiment of FIGS. 5A-5C eliminates the need for a collimating element, and the embodiment of FIG. 5C eliminates the need for a separate focusing mirror. Such a reduction in the number of components required allows for the manufacture of a low cost system due to both component elimination and assembly time reduction.

  As understood and appreciated by those skilled in the art, the approach shown in FIGS. 5A-5C can be applied to the embodiment of FIGS. 4A-4F for measurement of multiple bands of interest. For example, the components and arrangement of FIG. 5A can be advantageously used to modify the system of FIGS. 4B and 4D, and the components and arrangement of FIG. 5B can be advantageously used to modify the system of FIG. 4A. In each case, the focusing mirror is eliminated. The components and arrangement of FIG. 5B can also be used in the systems of FIGS. 4C, 4E, and 4F, although the focusing mirror and grating elements are more complex because they need to be focused in both reflection and transmission. The first or reflecting surface is a concave surface, and the second surface is a convex refractive surface.

  In the embodiment described above, the two different bands (ie, the 3.5-4.5 micron and 7-9 micron bands) are separately distributed using the first and second orders of the grating. Filters on the two detectors ensure that each detector reacts only to a unique band. The present invention also provides (essentially) non-adjacent bands that encompass a much wider range of wavelengths than can be achieved by a single order grating by using different orders of the grating. Intended.

  The invention described above discloses using a dichroic splitter to direct different bands or segments of bands to two different detectors. The present invention further contemplates the use of non-wavelength sensitive splitters (ie, conventional partial reflection splitters). In this case, an appropriate filter can be provided on or before the detector to sort the appropriate bands.

  The above embodiments of the present invention use a focusing mirror to form an image at the detector. This focusing function can also be performed with lenses formed from any suitable material. The present invention further contemplates that the dichroic splitter can be a partially reflective splitter. In addition, a splitter (reflective or transmissive) can be placed after the focusing mirror (or lens) and before the two detectors.

One function of the microspectrometer of the present invention is to simultaneously perform anesthetic spectral scans in the 8-10 micron IR band and mid-infrared CO ₂ and N ₂ O band scans. Numerous historical systems (eg Ebert, Czerny-Terner, Fastie-Ebert, etc.) and the presence of single or multiple holographic grating systems make it easy to select the basic structure for the spectrometer . A major system problem is efficiency, that is, the spectral resolution of the system with respect to the amount of light from the source that enters the detector.

  In all systems, the light source or aperture illuminated by the light source is imaged onto the sensor surface. The size of this image, defined by aberrations and optical magnification, must be less than the desired spectral resolution of the system. The effective light source size is important because the resolution is defined by the grating. In a typical spectrometer system, the entrance slit is at the focal point of a large aperture mirror. The mirror collimates the light onto the grating. The diffracted light from the grating is refocused on the sensor by the second concave mirror. Since the aperture is large (that is, the f value is small), the efficiency can be increased. In the microspectrometer of the present invention, light from the light source must first pass through an airway adapter (sample cell) that interferes with a large aperture / high efficiency system in the absence of additional optics. Even when the beam from the light source is collimated by the adapter, the size of the radiation source makes the beam divergence too large for practical optical components in the spectrometer.

  As shown in FIGS. 6A-6D, the present invention solves this problem by using a large circular lens at the light source that forms an aerial image in the center of the adapter (ie, sample cell). A lens 103 at the entrance to the detector block roughly collimates the light directly onto the grating. The lens 103 has a focal length that is approximately the same as the image distance of the lens adjacent to the light source. Lens 103 collimates the beam, and because it is functioning from the image of the light source, lens 103 helps to collimate the off-axis beam angle. This action is the same as that of the field lens. Therefore, the beam divergence at the grating is reduced and is much less in subsequent elements.

  6A and 6B, the diffracted light from the grating 106 is focused on the sensor (detector) 110 by the aspherical mirror 108. With this technique, the light source magnification is kept small and suitable for use, and the efficiency is high. The present invention contemplates that the lens be coated with silicon so that the lens is the cheapest lens material with reasonably good environmental stability for this wavelength band. In FIGS. 6C and 6D, a folding or reflecting mirror 109 is used in place of the concave focusing mirror.

  6A-6D show three other lens configurations. FIG. 6A shows an embodiment using a spherical lens 100 provided on one side of an adapter 102, also called a sample cell. FIG. 6B shows the use of an aspheric lens 104 with the adapter 102. FIG. 6C shows a focusing lens 107 provided in front of the folding mirror 109. FIG. 6D shows a focusing lens 111 provided after the folding mirror 109. The present invention also contemplates providing a focusing lens before and after the folding mirror. The remaining components of the system (eg, light source, reflective grating 106 and detector 110) can be configured in any arrangement contemplated by the present invention, including the specific embodiments discussed above.

Interest wavelength is from 4 to 4.7 microns to around 8 to 9.5 microns for CO ₂ and N ₂ O agents, and with respect to the reference channel 3.7 and 7.4 microns. The present invention contemplates that the same optical component and grating can scan both regions simultaneously, with the infrared region using the first order of the grating and the mid-infrared region using the second order of the grating. A dichroic splitter is required to separate the detectors.

The scanning rate of the grating 106 is preferably in the range of 100 Hz to 300 Hz. 100 Hz is an approximate lower limit set by the required CO ₂ bandwidth (ie, 10 Hz). The upper limit is defined by the mechanical constraints on the response time of the IR detector and the grating actuator. The range of motion of the spectrometer grating is (mechanically) ± 5 ° to cover the range including the reference channel, plus about 15% to 20% added for turn-around . If the reference function is performed in some other way or the grating spacing is reduced, the range of motion can be reduced to ± 3 °. In an exemplary embodiment of the invention, the grating mirror is approximately 6 mm wide and 10 mm high. These specifications are well within the cheapest state of the art at sinusoidal frequencies in the 200Hz-300Hz range. PbSe detectors are fast, sensitive and inexpensive and are well known and are used for the mid-infrared region. Candidate IR detectors are mercury cadmium tellurium (MCT), micro thermocouples, microbolometers or pyroelectric crystals.

Spectral data collected by the microspectrometer must include reference data regarding noise floor (zero signal), light source intensity (signal span, or clear channel), and spectrum span calibration. Calibration can be performed with reference to the CO ₂ line and edge filter. Calibration of either band or between bands is valid for both because the same scanner is used for both. Since the signal zero and span need to be implemented for each individual sensor, a clear channel and blocking function is required for each.

  The inventor has noticed that in operation, the scanner that rotates the diffraction grating operates at a single frequency and has a constant scan angle. These requirements have suggested to the inventor that a resonant scanner is a suitable system for driving a diffraction grating. The resonant scanner driving system has several advantages as follows. 1) Assuming a high mechanical quality factor, power requirements are minimized. 2) The scan operation tends to be an exact sine wave with the lowest harmonic. 3) An accurate synchronization signal can be derived from the drive circuit. The resonant scanner driving system is disadvantageous in that the resonant frequency depends on the inertia (mass) of the entire movable system and the magnitude of the restoring force (spring). If either changes with time, temperature or manufacturing variables, the resonant frequency changes.

  The hurdles faced when trying to use a resonant scanner drive system are that the inertia part other than the grating (as defined by the optical requirements) is minimized and the air intake is minimal (to maintain a high mechanical Q) Designing a system that is optimized and minimized in overall size. Furthermore, the system as a whole must accommodate some variation in resonance.

  The present invention addresses these issues and provides a scanner drive system 200 as shown, for example, in FIGS. 7A-9. The scanner drive system 200 includes a tote band 202 that provides a rotational axis for the diffraction grating 204. It should be noted that the diffraction grating has been omitted from FIG. 7A so that the features of the scanner drive system under the grating can be seen. Band 202 provides spring return and mechanical support to the movable components of the scanning system. The grating 204 is generally fixed in the center on one side of the band 202. A permanent magnet 206 is fixed on the opposite side of the band. Spacers 208 are provided on either side of the band 202 so that the twisted band does not contact the grating 204 or magnet 206 when the assembly oscillates.

  The band 202 is supported at its ends by a frame 210 that is square in the exemplary embodiment. During manufacture, the end of the band 202 is securely attached to the frame under tension. The present invention also contemplates holding the frame 210 under compression during the process so that the net tension in the band after attachment to the frame is predictable. The present invention contemplates attaching the band 202 to the frame 210 using spot welds, solder / brazing, or adhesive when the attachment is reinforced by bending the band beyond the outer edge of the frame. .

  In the exemplary embodiment, band 202 is 0.001 "thick, 0.9 mm wide and has a free length of about 7 mm. Grating substrate 2040 is glass, 2 mm thick and 6 mm in diameter. The resonant frequency is , Approximately 200 Hz, depending on the tension in the band and the proximity of the drive pole piece to the permanent magnet 206.

  Advantageously, the permanent magnet 206 is of the neodymium type that provides a particularly strong magnetic field for its size and mass. As indicated, the magnet is mounted on the band 2020 (by the spacer 208) so that the pole axis is perpendicular to the plane of the grating, ie the magnet surface attached to the tote band is a pole. It can be the North Pole or South Pole, but the condition must be defined during manufacture because the phase of oscillation (see below) relative to the drive pulse depends on the polarity of the magnet.

  The scanner is driven by magnetic interaction between the permanent magnet 206 and the nearby electromagnet 212. The electromagnet 212 has, in the exemplary embodiment, a “C” -shaped core 214 and a winding 216 of appropriate impedance is wound around the “C” central section. The electromagnet 212 can be regarded as a stator of an AC motor, and the permanent magnet 206 can be regarded as a rotor. The core 214 can be a laminated iron material, as in an audio transformer or an AC motor, or it can be a ferrite. Ferrite cores are relatively light weight and provide somewhat less eddy current loss, which in turn increases the mechanical Q value of the system. The electromagnet 212 is arranged so that the line between the two pole pieces is perpendicular to the axis of the band 202. The spacing between the magnet 206 and the electromagnet pole piece is not particularly important, but this clearance should not allow the magnet to contact the pole piece under any reasonable range of motion of the magnet. Otherwise, the magnet will stick to the pole piece and the system will stop.

  The electrical drive provided to the electromagnet 212 is in the form of short pulses. The scanner assembly “rings” at the mechanical resonance frequency. Since the Q value is high (in the range of 100 to 150), the oscillation reaches amplitude balance with a few pulses.

  In a typical resonant system, the drive delays operation by an amount close to 90 ° depending on the mechanical loss. The system has little loss and the drive pulse is at the maximum speed point (ie 90 °) at resonance. Since the system is both a motor and a generator, any movement of the magnet generates a return voltage in the electromagnet coil.

  During resonance, the return signal can be shown or visualized as a sine wave 220 as shown in FIG. 9A. The sine wave 220 includes a spike 222 that is a drive pulse in the center of the sine wave. A sine wave 220 was generated by attaching an oscilloscope directly to the drive coil, and a resistor was used to partially isolate the drive pulse electronics from the return signal. When the drive pulse rate is not in resonance, the return signal phase is different from that shown in FIG. 9A. FIG. 9B shows the return signal 224 when the system is not in resonance. It will be appreciated that spike 225 is not centered on the peak of the sine wave but is offset or off-center. By comparing the return signal immediately before the drive pulse with the signal immediately after the drive pulse, the phase error can be converted into a signal that can be used to adjust the drive frequency.

  A block diagram circuit 230 for performing this automatic frequency adjustment is shown in FIG. A voltage controlled oscillator (VCO) 232 provides a time reference to the system. It has a nominal frequency close to mechanical resonance. The pulses from the VCO are supplied to a 3-bit binary counter 234 that drives a 3-bit decode 236. The resulting 8 time series signals are used to control the system.

  Referring to the sinusoidal diagram 240 shown in FIG. 8, the first two periods are used to collect data from the signal before the drive pulse, and the fourth and fifth periods are from the signal immediately after the pulse. Collect data. Data is collected in capacitor C1 via sample hold (S / H) 242. A drive pulse is generated in period 3.

  During periods 6 and 7, this time, in time synchronization, the signal difference is transmitted to capacitors C2 and VCO232. During period 8, each capacitor C1 is discharged slightly to aid in loop response time. Period 3 drives a transistor 244 such as a MOSFET that injects current through the isolation resistor R into the excitation coil 216.

  The amplitude of the return signal is proportional to the peak velocity that is proportional to the maximum scan angle for a given frequency. Thus, the return signal amplitude is used to provide feedback to the drive pulse size, thereby maintaining a constant scan angle. The negative half cycle of the return signal is used for this purpose. Diode D1 and capacitor C3 provide the return signal average voltage to the differential amplifier. A constant set point is supplied to the opposite side of the amplifier. The amplified difference is the pulse amplitude.

  The present invention also contemplates that the coil and pole structure can be rotated about an axis defined by the pole tip. In other words, the assembly can be folded relative to the frame. Such a change shortens the scanner assembly, but makes it slightly wider in one direction. The present invention contemplates that two separate windings can be placed on the electromagnet core. The two windings provide better impedance matching for the driver and separately for the return amplifier. Since the signal is floating, it further improves the S / N of the return signal.

  Although the grating is shown as a disk in FIG. 7B, other shapes such as squares or rectangles are contemplated. The important point is that the spectral resolution of the spectrometer is in part proportional to the width of the grating, ie the number of grating grooves in the light beam. In manufacturing, it is advantageous to make the grating rotationally asymmetric so that the grating groove is indeed mounted parallel to the axis of rotation, ie the long axis of the tote band.

  The frame is expected to be the element that provides the main strength of the scanner assembly, and therefore it is the element that is fixed to the spectrometer system. The frame is shown as a square. However, it can have other shapes (eg, circular or any combination of shapes) and can include mounting protrusions or brackets.

  While the spectrometer of the present invention has been described in detail for purposes of illustration, based on what is presently considered to be the most practical and preferred embodiment, such details are merely for that purpose. It is to be understood that the invention is not limited to the disclosed embodiments, but on the contrary is intended to cover modifications and equivalents that are within the spirit and scope of the appended claims. It is.

Claims (6)

An infrared light source that emits an infrared beam,
A gas sample cell disposed in the path of the infrared beam;
A scanning mirror having a diffraction grating having a plurality of parallel lines disposed in the path of the infrared beam after passing through the gas sample cell;
A resonant scanner drive system for oscillating the scanning mirror about an axis parallel to the line of the diffraction grating;
A first focusing element arranged to focus at least one band of interest of the infrared beam diffracted by the diffraction grating;
A first detector arranged to receive the focused at least one band of interest and a first detector readout operatively coupled to the first detector to receive a signal from the first detector; circuit,
Spectrometer with A splitter disposed in the path of the diffracted infrared beam from the scanning mirror;
A second focusing element disposed in each path of the discrete band of interest;
A second detector arranged to receive a focused discrete band of interest, and a second detector readout circuit operatively coupled to the second detector to receive a signal from the second detector;
The spectrometer according to claim 1, further comprising:   The spectrometer according to claim 2, wherein the splitter is a dichroic splitter for separating the diffracted infrared beam into discrete bands of interest.   The spectrometer according to claim 2, wherein the first focusing element is a lens or a mirror, and the second focusing element is a lens or a mirror.   The spectrometer according to claim 1, wherein the first focusing element is a lens or a mirror.   The spectrometer according to claim 1, further comprising a large aperture lens disposed between the infrared light source and the sample cell.

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